It is well known that the optical characteristics of an eye can be altered through laser surgery. For example, U.S. Pat. No. 6,050,687 which issued to Bille et al. for an invention entitled “Method and Apparatus for Measuring the Refractive Properties of the Human Eye,” discloses a laser system that can be used for such purposes. In any event, a consequence of photoablation, is that individual cells in the tissue are vaporized. Gas is, therefore, a product of photoablation. When a surgical laser procedure involves the superficial photoablation of tissue, the fact that such gases are created does not cause much of a problem. This is not the case, however, when internal tissue is photoablated.
For specific surgical procedures that involve the intrastromal photoablation of corneal tissue, it is known that such photoablation results in the formation of tiny bubbles in the stroma. Further, it is known that the formation of these bubbles introduces aberrations that change the optical characteristics of the cornea. The reason for this change is essentially two-fold. First, the gas bubbles have a different refractive index than that of the surrounding stromal tissue. Second, and perhaps more important, the gas bubbles tend to deform the stroma and, thus, they alter its refractive effect on light passing through the cornea. In a controlled surgical procedure these induced aberrations must be accounted for.
Wavefront analysis provides a useful and helpful conceptual tool for determining the effect a particular medium or material (e.g. the cornea of an eye) will have on a light beam, as the beam passes through the medium (material). For wavefront analysis, a light beam can be conveniently considered as being a so-called “bundle” of component light beams. These component light beams are all mutually parallel to each other, and when all of the component beams of a light beam are in phase with each other as they pass through a plane in space, it is said they define a plane wavefront. However, when a light beam passes through a medium, the medium will most likely have a different refractive effect on each of the individual component beams of the light beam. The result is that the phases of the component light beams will differ from each other. When now considered collectively, these component light beams will define something other than a plane wavefront. In summary, a particular wavefront will define the refractive effect a medium, or several media, have had on a light beam.
Insofar as laser surgery is concerned, it is the objective of such surgery to remove unwanted aberrations from the light beams that a patient perceives visually. As implied above, wavefront analysis can be a helpful tool in evaluating and determining the extent to which refractive properties of a cornea may need to be altered or corrected. Indeed, such an analysis has been helpful for surgical procedures involving superficial photoablation. For example, U.S. Pat. No. 6,428,533B1 which issued to Bille for an invention entitled “Closed Loop Control for Refractive Laser Surgery (LASIK),” and which is assigned to the same assignee as the present invention, discloses such a system.
As recognized by the present invention, when intrastromal photoablation is to be performed, and the evaluations and determinations of a wavefront analysis are put into practice, it is desirable to establish control over each individual component beam defining a wavefront. With this control, induced aberrations such as the aberrations mentioned above, can be accurately compensated for, and the overall control of the procedure enhanced.
As further recognized by the present invention, the temporal effect of intrastromal photoablation can be characterized as having three distinct periods. The first period, which typically lasts for about 30-60 minutes after tissue photoablation, is characterized by the presence of a gas bubble in the stroma. The second period, on the other hand, occurs after the gas bubble has collapsed. During this second period, photoablation induced stresses remain in the stromal tissue that previously surrounded the gas bubble. These induced stresses relax during the second period, and, during this relaxation, the curvature of the cornea changes. Eventually, the curvature of the cornea stabilizes. Thus, the onset of the third period corresponds to the time when the induced stresses have relaxed and the shape of the cornea has substantially stabilized. This stable shape, in turn, represents the long-term surgical performance of the ablation. Typically, the third period begins approximately 1-30 days after the ablation.
In general, a single procedure for corneal corrections can require hundreds of intrastromal photoablations, each of which results in the formation of a corresponding gas bubble. For these corneal corrections, a typical laser pulse repetition rate of approximately 10 kHz is used. With this pulse repetition rate, it can be appreciated that some ablations will occur during a procedure before all of the previously created bubbles have collapsed. Indeed, it is typical for the entire procedure to be completed before any of the gas bubbles have fully collapsed. With the above in mind, it is possible to measure a wavefront between successive ablations which is indicative of corneal shape. This real-time information can then be used to update a treatment plan and thereby revise the planned position and size of subsequent ablations during a procedure.
In a complicated procedure such as a corneal reshaping, it would be helpful to predict the long term surgical effect of an ablation (i.e. the eventual corneal shape after bubble collapse and relaxation of photoablation induced stresses). Preferably, this prediction could be made in real-time from a wavefront measurement that is obtained during a procedure (i.e. after an ablation but before the resulting bubble has collapsed). This information could then be used to modify a treatment plan during the procedure, to increase the overall accuracy of the procedure. As recognized by the present invention, however, the relaxation of induced stresses which occurs during the second period described above, and its effect on corneal shape, is somewhat unpredictable. At least this is so using current models. On the other hand, the inventors have determined that it is possible to estimate the corneal shape after bubble collapse (i.e. at the end of the second period described above). Moreover, this can be done with real-time wavefront data measured between successive bubbles during a procedure.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of using real-time wavefront data measured during an intrastromal photodisruption procedure to improve the surgical performance of the procedure. It is another object of the present invention to provide systems and methods for performing an accurate optical correction which accounts for the relatively long-term effects associated with the relaxation of photoablation induced stresses. It is yet another object of the present invention to provide systems and methods for intrastromal photoablation which are easy to use, relatively simple to implement, and comparatively cost effective.